The tunnel cuts through a portion of Mount Chiliad and connects the Paleto Forest to the Alamo Sea area. The tunnel sits in a somewhat isolated location and stands out around the natural surroundings. The only route that goes through the tunnel is a rural road, which is part of a larger dirt road running through Paleto Forest, and the dirt road leads to the Paleto Sawmill. This tunnel was most likely constructed to aid in the transport of lumber and other materials from the Paleto Sawmill to other parts of the state. The roof of the tunnel is lined with large lights.
Inside the tunnel, there are two service doors that are marked T01 and T02. These two same numbers appear, coincidentally or not, on the rooftops of the FIB and IAA Buildings in Los Santos. This leads some to believe that the tunnel is associated with the FIB and IAA. Also, a storm drain near the tunnel's northern entrance is claimed to somehow be enterable, and that the storm drain and the service doors lead to a secret chamber inside Mount Chiliad. The Mount Chiliad Mystery and the hunt for the hidden Jetpack in the game make many believe that the tunnel is hiding something. So far, the storm drain and the service doors are unenterable, and a hidden chamber in Mount Chiliad has yet to be proven. The tunnel serves no purpose in the storyline. However it is related to a mission in GTA Online that is the likely conclusion of the Mount Chiliad Mystery.
There have been reports of strange whispers and voices, none of which have been confirmed yet, as most reports have little to no evidence of the whispers or ghostly presence of any sort. There have also been reports of players feeling uneasy, anxious, and even having the feeling of being watched. These reports vary from person to person, but it is all up for interpretation as of yet.